One of the methods for manufacturing photodiodes in semiconductor materials with a small band gap (often for infrared light detection) consists of inserting the detection active small band gap layer between two large gap semiconductor materials. Both large band gap semiconductor layers are an efficient protection/passivation while remaining transparent to the wavelength of the radiation intended to be detected by the photodiode.
Further, with suitable dopings, both heterojunctions between the active layer and the two protection/passivation layers confine photoelectric charges in the active detection layer and improve the quantum yield of the thereby built photodiode.
An InGaAs photodiode is a typical example of this critical structure. The detection active layer consisting of InGaAs material may have an adjustable band gap depending on the indium, and gallium composition in InGaAs, ideal for operating in the SWIR (Short Wave Infra-Red) band of the order of 1.4 to 3 μm.
Indium phosphide and indium-gallium arsenide have the same face centered cubic crystalline structure. The most used composition is In0.53Ga0.47As. The crystalline lattice size is then comparable with that of the InP substrate, notably the lattice parameters. This crystalline compatibility allows growth by epitaxy of an active InGaAs layer of excellent quality on an InP substrate. The band gap of In0.53Ga0.47As is of about 0.73 eV, capable of detecting up to a wavelength of 1.68 μm in the SWIR band. It has a growing benefit in fields of applications such as spectrometry, night vision, sorting of used plastics, etc.
Both protection/passivation layers are generally made in InP. Especially as the composition In0.53Ga0.47As, has the same crystalline lattice size as InP, this allows a very low darkness current from room temperature onwards.
FIG. 1 illustrates the physical structure of an array 1 of photodiodes. An active layer 5 consisting of InGaAs is sandwiched between two InP layers. The lower layer actually forms the substrate 4 on which the InGaAs layer is formed by complex MO-CVD epitaxy. This InGaAs layer is then protected by a thin passivation layer 6 consisting of InP, also deposited by epitaxy. the InP layers are generally of the N type, doped with silicon. The active layer 5 of InGaAs may be slightly N-doped or may remain quasi-intrinsic. So both lower/upper InP layers and the active InGaAs layer 5 form the common cathode of the photodiodes in this array.
The individual anodes 3 are formed by local diffusion of zinc (Zn). The dopant Zn crosses the thin passivation InP layer 6 and penetrates into the active InGaAs layer 5.
FIG. 2 illustrates an InGaAs image sensor consisting of an array 1 of InGaAs photodiodes connected in a flip-chip mode with a readout circuit 2. In an InGaAs array sensor, the photodiode array is connected to a readout circuit generally made in silicon in order to read the photoelectric signals generated by the InGaAs photodiodes. This interconnection is generally achieved with the flip-chip process via indium beads 7, as illustrated in FIG. 2. The SWIR radiation 9 arrives on the photodiode array through the indium phosphide substrate 4, transparent in this optical band.
With a detector operating in an integration mode, an output signal is obtained proportional to the product of the flux and of the exposure time. However, the output signal is limited by the maximum integration capability of the sensor. For scenes with high contrast, it is often impossible to obtain good rendering of dark areas and at the same time to keep bright areas without any saturation. This problem is all the more serious for night vision for which an array sensor with InGaAs photodiodes is often intended.
Another way of reading the photoelectric signals from photodiodes, in a general way, is proposed by document EP 1 354 360 and illustrated in its principle by FIG. 3 of the drawings appended herein. Document EP 1 354 360 proposes a solar cell operating mode of a photodiode in order to obtain a logarithmic response versus the intensity of the incident optical radiation 59.
In this operating mode, the photodiode 51 does not receive any external bias and it is forward-biased by the photoelectric charges generated in its junction. The direct bias voltage observed on the photodiode is proportional to the logarithm of the incident light flux.
This logarithmic response gives the possibility of covering without any electrical and optical adjustment, an operating dynamic range of more than 120 dB, indispensable for using a SWIR InGaAs sensor under natural conditions outdoors. Document EP 1 354 360 also proposes association of a switching readout circuit 55 with the photodiode.
The principle of use of the image sensor array illustrated in FIG. 3 is the following:                a) The selection signal SEL is enabled in order to select the desired photodiode 51 by closing the switch 54. Once this photodiode is selected, the first readout signal RD1 is enabled which will close the corresponding controlled switch with the purpose of storing in memory the voltages from a first readout in the memory 56. This first readout records both the image and fixed spatial noise.        b) The reset signal RSI is then enabled, a signal will cause closing of the switch 53. The photodiode 51 thus being short-circuited, a reference image in absolute darkness is thus simulated.        c) The first readout signal RD1 is then disabled in order to reopen the corresponding switch and the second readout signal RD2 is then enabled for thus recording in the memory element 57 the voltages of the second readout. The fixed spatial noise has thus been stored in memory alone.        d) The difference between the result of both memory storages contained in the respective memory elements 56 and 57 is then calculated by a differential amplifier 58. The output signal of this amplifier 58 then corresponds to an image free of fixed spatial noise.        
By means of the second readout, zero voltage corresponding to the darkness condition is generated. This electronic darkness signal gives the possibility of suppressing signal offsets in the readout chain in an array detector.
The principle proposed by EP 1 354 360 was applied in an InGaAs sensor and operates perfectly. But a blooming phenomenon is observed for daylight scenes. This phenomenon may simply be described as a loss of spatial resolution in an image. The detector nevertheless continues to be sensitive to variation of light according to the logarithmic law. This phenomenon is not observed in other types of photodiodes such as those based on silicon, InSb or MCT.